The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in the high-tech electronic industries. Studies on lithium secondary batteries are actively being pursued in accordance with the increased need for batteries having high energy density for use as the power sources in these portable electronic instruments. Materials that are capable of reversible intercalation/deintercalation of lithium ions such as lithium-transition metal oxides are used as a positive active material of a lithium secondary battery, and lithium metals, lithium-containing alloys, or materials that are capable of reversible intercalation/deintercalation of lithium ions such as crystalline or amorphous carbons, or carbon-containing composites, are used as a negative active material of a lithium secondary battery.
A cross-sectional view of a general non-aqueous Li-ion cell is shown in FIG. 1. The Li-ion cell 1 is fabricated by inserting an electrode assembly 8 including a positive electrode 2, a negative electrode 4, and a separator 6 between the positive and negative electrodes into a battery case 10. An electrolyte 26 is injected into the battery case 10 and impregnated into the separator 6. The upper part of the case 10 is sealed with a cap plate 12 and a sealing gasket 14. The cap plate 12 has a safety vent 16 to release pressure. A positive electrode tab 18 and a negative electrode tab 20 are respectively attached to the positive electrode 2 and negative electrode 4. Insulators 22 and 24 are installed on the lower part and the side part of the electrode assembly 8 to prevent a short circuit occurrence in the battery.
The average discharge voltage of a lithium secondary battery is about 3.6 to 3.7V, which is higher than other alkali batteries, Ni-MH batteries, Ni—Cd batteries, etc. An electrolyte that is electrochemically stable in the charge and discharge voltage range of 0 to 4.2V is required in order to generate such a high driving voltage. As a result, a mixture of non-aqueous carbonate-based solvents, such as ethylene carbonate, dimethyl carbonate, diethyl carbonate, etc., is often used as an electrolyte. However, such an electrolyte has significantly lower ion conductivity than an aqueous electrolyte that is used in a Ni-MH battery or a Ni—Cd battery, thereby resulting in the deterioration of battery characteristics during charging and discharging at high rate.
During the initial charge of a lithium secondary battery, lithium ions, which are released from the lithium-transition metal oxide positive electrode of the battery, are transferred to a carbon negative electrode where the ions are intercalated into the carbon. Because of its high reactivity, lithium reacts with the carbon negative electrode to produce Li2CO3, LiO, LiOH, etc., thereby forming a thin film on the surface of the negative electrode. This film is referred to as an organic solid electrolyte interface (SEI) film. The organic SEI film formed during the initial charge not only prevents reaction between lithium ions and the carbon negative electrode or other materials during charging and discharging, but it also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents the disintegration of the structure of the carbon negative electrode, which causes organic solvents in an electrolyte with a high molecular weight to solvate lithium ions, and the solvent and the solvated lithium ions co-intercalate into the carbon negative electrode.
Once the organic SEI film is formed, lithium ions do not react any further with the carbon electrode or other materials such that the amount of lithium ions is maintained. That is, the carbon of the negative electrode reacts with the electrolyte during the initial charging, thus forming a passivation layer such as an organic SEI film on the surface of the negative electrode such that the electrolyte solution no longer decomposes, and stable charging and discharging are maintained (J. Power Sources, 51 (1994), 79-104). For these reasons, in a lithium secondary battery, there is no irreversible formation reaction of the passivation layer, and a stable cycle life after the initial charging reaction is maintained.
For a thin prismatic battery, gases are generated inside the battery due to decomposition of a carbonate based organic solvent during the organic SEI film-forming reaction (J. Power Sources, 72 (1998), 66-70). These gases include H2, CO, CO2, CH4, C2H6, C3H8, C3H6, etc. depending on the type of non-aqueous organic solvent and negative active material used. The thickness of the battery increases during charging due to the generation of gas inside the battery, and the passivation layer slowly disintegrates by electrochemical energy and heat energy, which increase with the passage of time when the battery is stored at high temperatures after it is charged. Accordingly, a side reaction in which an exposed surface of the negative electrode reacts with surrounding electrolyte occurs continuously.
The above problems occur in a positive electrode. At initial charging, positive active material reacts with electrolyte to form a passivation layer on the positive electrode, and the passivation layer prevents decomposition of electrolyte resulting in maintenance of stable charge-discharge. For the negative electrode, the charge consumed during formation of the passivation layer on the positive electrode is irreversible. For this reason, in a lithium ion battery, there is no irreversible formation reaction of the passivation layer, and a stable cycle life after the initial charging reaction is maintained.
However, as mentioned previously, the passivation layer slowly disintegrates by electrochemical energy and heat energy, which increase with the passage of time when the fully charged battery is stored at high temperatures after it is charged. For example, if a battery is stored at 85° C. for four days after a 100% charge at 4.2 V, an exposed surface of the positive electrode reacts with surrounding electrolyte to generate gases. The generated gases include CO, CO2, CH4, C2H6, etc. from decomposition of a carbonate-based solvent.
The internal pressure of the battery increases with this generation of gases in both positive and negative electrodes. The increase in the internal pressure induces the deformation of prismatic and lithium polymer batteries. As a result, regional differences in the cohesion among electrodes inside the electrode assembly (positive and negative electrodes, and separator) of the battery occur, thereby deteriorating the performance and safety of the battery and making it difficult to mount the lithium secondary battery set into electronic equipment.
Further, disintegration of the passivation layer due to an increase of electric or thermal energy results in a continuous side reaction between positive and negative electrodes and the electrolyte. Gases generated from the side reaction increase internal pressure inside battery causing deformation of the battery to induce a short or thermal runaway.
As one method for solving the internal pressure problem, there is disclosed a method in which the safety of a secondary battery including a non-aqueous electrolyte is improved by mounting a vent or a current breaker for ejecting internal electrolyte solution when the internal pressure is increased above a certain level. However, a problem with this method is that mis-operation may result from an increase in internal pressure itself.
Furthermore, a method in which the SEI-forming reaction is changed by injecting additives into an electrolyte so as to inhibit the increase in internal pressure is known. For example, Japanese Patent Laid-open No. 97-73918 discloses a method in which high temperature storage characteristics of a battery are improved by adding 1% or less of a diphenyl picrylhydrazyl compound to the electrolyte. Japanese Patent Laid-open No. 96-321312 discloses a method in which cycle life and long-term storage characteristics are improved using 1 to 20% of an N-butyl amine based compound in an electrolyte. Japanese Patent Laid-open No. 96-64238 discloses a method in which storage characteristics of a battery are improved by adding 3×10−4 to 3×10−2 M of calcium salt to the electrolyte. Japanese Patent Laid-open No. 94-333596 discloses a method in which storage characteristics of a battery are improved by adding an azo-based compound to inhibit the reaction between an electrolyte and the negative electrode of the battery. In addition, Japanese Patent Laid-open No. 95-320779 discloses a method in which CO2 is added to an electrolyte, and Japanese Patent Laid-open No. 95-320779 discloses a method in which sulfide-based compounds are added to an electrolyte in order to prevent the electrolyte from decomposing.
Such methods as described above for inducing the formation of an appropriate film on a negative electrode surface such as an organic SEI film by adding a small amount of organic or inorganic materials are used in order to improve the storage characteristics and safety of a battery. However, there are various problems with these methods: the added compound often decomposes or forms an unstable film by interacting with the carbon negative electrode during the initial charge and discharge due to inherent electrochemical characteristics, resulting in the deterioration of the ion mobility in electrons; and gas is generated inside the battery such that there is an increase in internal pressure, resulting in significant deterioration of the storage, safety, cycle life, and capacity characteristics of the battery.